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| Book/Report | FZJ-2018-03944 |
1992
Forschungszentrum Jülich GmbH Zentralbibliothek Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/19230
Report No.: Juel-2675
Abstract: Frequently, the mechanism of phase transformations cannot be determined on the basis of equilibrium investigations alone. More detailed information is obtained if the evolution of a system is studied during a change of state. Time-resolved nonequilibrium experiments yield informations about possible intermediate states andabout the question how structural changes are realized. In the present paper, two model systems are investigated in detail: Phase separation in ionic crystals isstudied using the system AgBr-NaBr, while transitions between modulated phases are investigated in tetramethylammonium-tetrachlorozincate ([N(CH$_{3}$)$_{4}$]$_{2}$ZnCl$_{4}$), commonly denoted as TMATC-Zn. In both cases, neutron scattering is used in order to characterize the microscopic state of the sample. Whereas the large information content is one of the essential advantages of neutron scattering, the weak interaction of neutrons with matter and the relatively low intensity of neutron sources provide its most severe disadvantage. Hence, in the past real-time experiments with neutrons were restricted to a small number of special problems. The present kinetic investigations with a time-resolution down to fractions of a second, therefore, required the development of a novel technique: time-resolved triple-axis spectroscopy. Here, the sample is perturbed periodically by some external field, temperature in the present case. Consequently, the scattered intensity is not only recorded as a function of momentum and energy transfer but also as a function of time with respect to the external perturbation. Provided the process under consideration is strictly reversible, scattering intensity can be accumulated over several periods. Thus, a time-resolution in the millisecond range can be achieved by this stroboscopic method even if total counting times are typically of the order of minutes. The kinetics of decomposition in the system AgBr-NaBr can be monitored by the profiles of Bragg-reflections from the very beginning up to the final state of complete phase separation. Due to the different lattice parameters of AgBr and NaBr, these data reflect the instantaneous concentration distribution within the sample. Usually, kinetic studies of decomposition are concerned with the determination of grain size distributions, especially in the earlier stages. Hence, the present approach yields complementary information characterizing the whole decomposition process quantitatively by time-dependent concentration distributions. in this paper, a simple and quite general mathematical model is developed which is able to reproduce all of the experimental results surprisingly well. This model is based on a non-linear master-equation for the concentration distribution; it revealed that the phase separation is governed by a double-minimum potential. in addition, however, mechanical stresses play an important role even 150 K below the critical temperature. The corresponding coherency strains seem to be responsible for the pronounced delay of the decomposition at higher temperatures. Moreover, coarsening effects which are frequently investigated, both theoretically and experimentally, influence the time evolution of the concentration distribution. Transitions between commensurately or incommensurately modulated structures are studied in the system TMATC-Zn. The results of these investigations are compared to computer simulations which predict the existence of intermediate states during the phase transformations due to topological defects like stripples or planar nuclei. For the first time, satellite spectra are observed experimentally as a function of time under the influence of fast temperature changes. There is no diffuse scattering, neither during the lock-in transition nor during the transition between the commensurate 2/5- and 1/3-phases. Hence, there are no indications for any intermediate state. in particular, a sequence of transient commensurate phases corresponding to a devil's staircase can be ruled out. Rather, there is a definite switching between well-defined phases. Moreover, detailed investigations under different experimental conditions prove that the kinetics of the lock-in transition, in particular, is strongly affected by internal strain fields, produced during fast temperature changes. This effect was not taken into account in the computer-simulations. It leads to the stabilization of a considerable amount of the commensurate phase even at temperatures well above the lock-in transition. Both examples demonstrate that time-resolved non-equilibrium experiments yield interesting and novel information about the course and the mechanism of phase transitions. A variety of different systems may be studied by this method. The determination of non-equilibrium states induced by electric or magnetic fields, forinstance, could be one attractive problem. External parameters of this type can, perhaps, be varied in a more defined way than the temperature. Moreover, thekinetics of domain distributions or the relaxation of disordered states could be studied on a microscopic level. Time-dependent relaxation processes in glassy systemsor the kinetics of (reversible) solid state reactions are other fascinating subjects. The present method of time-resolved triple-axis spectroscopy is, however, not restricted to structural investigations. Rather, time-dependent changes of dynamical properties, like the phonon-dispersion near soft-mode transitions or the redistribution of phonon states, can be studied equally well in real-time experiments. Investigations of systems far from equilibrium yield valuable informations about the reasons why a particular state is realized in equilibrium; if microscopic properties of systems are determined under non-equilibrium conditions and compared to those properties which are found in equilibrium, the driving force for a transformation or the cause of the stability of a particular phase should show up clearly.
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